The myxobacteria are a group of Gram-negative bacteria that undergo a true multicellular development and primitive differentiation. They provide a useful system for investigating cell-cell interaction and development in a simple organism because they resemble more familiar bacteria in structure and can be easily grown, manipulated and studied in genetically pure form. Myxobacteria grow vegetatively like other bacteria, but when nutrients are limiting myxobacteria aggregate to form fruiting bodies. Within the fruiting bodies, the cells sporulate, changing from long vegetative rods to short ovoid spores with refractory properties. Under appropriate conditions, the spores germinate to give vegetative cells once more. Our objectives are to identify the signals that myxobacteria exchange when they interact with each other to form fruiting bodies, to discover the regulatory switch that turns fruiting on, and to locate the cell surface structures required for fruiting.